1. Field of the Invention
The present invention relates to an improvement of a control apparatus for a drive system of a hybrid vehicle.
2. Description of Related Art
There is proposed a technique for switching a hybrid vehicle drive system provided with an electric motor functioning as a vehicle drive power source in addition to an internal combustion engine, between a hybrid drive mode and an EV drive mode. In the hybrid drive mode, the engine and the electric motor are operated as the vehicle drive power source to drive the hybrid vehicle. In the EV drive mode, the electric motor is operated as the vehicle drive power source to drive the hybrid vehicle while the engine is held at rest. JP-2011-156899 A discloses an example of such a hybrid vehicle drive system, which is preferentially placed in the EV drive mode when an amount of an electric energy stored in a battery is comparatively large, whereby deterioration of drivability of the hybrid vehicle can be prevented. Further, JP-2011-098712 A discloses an example of a hybrid vehicle drive system including: a differential device which has a first differential mechanism and a second differential mechanism and which has four rotary components; and an engine, a first electric motor, a second electric motor and an output rotary member which are respectively connected to said four rotary components. This hybrid vehicle drive system is configured to be switched to a selected one of a plurality of hybrid drive modes and a plurality of EV drive modes, according to a selected one of a plurality of combinations of the operating states of a plurality of coupling elements.
Where the hybrid vehicle drive system is switched to the selected one of the drive modes on the basis of a required vehicle drive force and the amount of the electric energy stored in the battery, as in the prior art described above, the frequency of change of the operating states of the coupling elements from one combination to another increases with the number of the vehicle drive modes available, so that a drive mode switching control of the hybrid vehicle drive system suffers from a time lag or a low degree of drive mode switching response, giving rise to a problem of deterioration of the drivability of the hybrid vehicle. This problem was first discovered by the present inventors in the process of an intensive study in an effort to improve the performance of the hybrid vehicle.